My first guess would be that there isn't enough of a gap between the flipper and the bushing. As you play, the friction creates heat which causes the bushing to slightly expand resulting in a slow flipper return.

could also be that your return spring fell off or is stretched out.

-c

Inspect it and make sure it looks like the other flipper mechanism to see if something is missing or broken. If that doesn't work, take it apart and clean it.

You have to loosen them, properly adjust the gap, and then tighten them back up. The gap should be equal to about the thickness of a credit card. Carefully read through this thread:

http://pinside.com/pinball/forum/topic/vids-guide-to-upgradingrebuilding-flippers

You will find it very enlightening.

Push the flipper foreward by hand and let it go with the machine OFF. They should both drop right back. If they do you are looking at something electrical. If one drops back slow with no power on the pin you have a mechanical issue. The shaft of the flipper could be dragging or dirty. This will make the flipper slow returning. It could also be a worn bushing in the pf.